Entangled photons are an essential resource for quantum key distribution over large distances and scalable linear optics for quantum computing. When two photons are entangled, both photons occupy a state which is a superposition of two states. Measurement of one photon causes the wave function to collapse which in turn sets the state for the other photon. The entanglement extends over long distances. Current entangled light sources concentrate on the time average relationship between paired photons. However, entanglement can evolve over the duration of a photon wave package and light states can show strong quantum correlations at an instant but can appear to be classical when averaged over time.
The inventors have surprisingly realised that many states which have previously been regarded as classical can in fact show entanglement when resolved as a function of time.